Water Vapor Turbulence Profiles of the Tropical Convective Boundary Layer
ARM Raman lidar data provide insights into turbulence structure in the tropics.
The lowest portion of the atmosphere that is directly influenced by its contact with the Earth’s surface is called the atmospheric boundary layer. During the daytime, solar heating of the surface drives convective mixing and turbulence in the convective boundary layer. Accurately representing turbulence in numerical models is critical because turbulence mixes mass, momentum, and energy within the convective boundary layer. Since turbulence works on a range of scales it must be parameterized in these models. This study examines detailed profiles of water vapor turbulence using the Raman lidar observations from the Atmospheric Radiation Measurement (ARM) Tropical Western Pacific (TWP) site located at Darwin, Australia in order to calculate statistical moments of the turbulence profiles that are often used in turbulence parameterizations. These were compared to statistics from a similar Raman lidar at a midlatitude site, the ARM Southern Great Plains site in Oklahoma.
This study presented the first vertical profiles of water vapor turbulence from a ground-based lidar at a tropical site. The study found striking differences in these variables among the tropical wet season, tropical dry season, and midlatitude cases. The tropical wet season differed greatly from the dry season, while the tropical dry season and the SGP site were more similar. The main drivers of this marked seasonality difference in the profiles are the moist maritime air masses that come from the ocean as a part of monsoonal atmospheric circulation in northern Australia and also the relatively strong mixing in the entrainment zone during the wet season. This study demonstrates the value of the continuous, long-term, high temporal, and vertical resolution observations of water vapor from the ARM Raman lidars. The unique data set of the profiles of turbulent statistics presented here can be used for validation of similarity relationships (often used in boundary layer parameterizations), which have traditionally been evaluated only by large eddy model simulations.
This study explored water vapor turbulence in the convective boundary layer using the Raman lidar observations from the Atmospheric Radiation Measurement site located at Darwin, Australia. An autocovariance technique was used to separate out the random instrument error from the atmospheric variability during time periods when the convective boundary layer is cloud-free, quasi-stationary, and well mixed. The study identified 45 cases, comprising of 8 wet and 37 dry seasons events, over the 5-year data record period. The dry season in Darwin is known by warm and dry sunny days, while the wet season is characterized by high humidity and monsoonal rains. The inherent variability of the latter resulted in a more limited number of cases during the wet season. Profiles of the integral scale, variance, coefficient of the structure function, and skewness were analyzed and compared with similar observations from the Raman lidar at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site. The wet season shows larger median variance profiles than the dry season, while the median profile of the variance from the dry season and the SGP site are found to be more comparable. The variance and coefficient of the structure function show qualitatively the same vertical pattern. Furthermore, deeper convective boundary layer, larger gradient of water vapor mixing ratio at height of maximum variance, and the strong correlation with the water vapor variance are seen during the dry season. These continuous, long-term, high temporal and vertical resolution observations of water vapor are valuable to evaluate the performance of turbulence parameterization schemes in models in the convective boundary layer, which are essential for improved weather forecasts, regional climate projections, and simulating convection initiation and the formation of clouds and precipitation.
M. K. Osman
The University of Oklahoma and NOAA/National Severe Storms Laboratory
This work was supported by the U.S. Department of Energy Atmospheric System Research (ASR) program via grant DE-SC0014375. The data used were collected as part of the Atmospheric Radiation Measurement (ARM) program and are available from the ARM data archive at http://www.arm.gov.
Osman M, D Turner, T Heus, and R Newsom. “Characteristics of Water Vapor Turbulence Profiles in Convective Boundary Layers During the Dry and Wet Seasons over Darwin.” Journal of Geophysical Research: Atmospheres, 123(10), 4818-4836 (2018). [DOI:10.1029/2017JD028060]